I was just wondering why there is bleed air in the cabin and not ram air. during the last weeks the media often mentioned the problem of oil leakage in the engines that could cause oil vapor to be brought into the cabin.
it would just need a second air cycle machine to transfer the energy from the bleed air to the ram air

this is a schematic of a common pack:

I edited it to visualize my idea. maybe it´s a bit confusing on the first view

I guess the additional weight of the second air cycle machine would equal the weight of a compressor needed for a bleed-less pack like the one of the B787

instead of. don´t bring bleed air to the cabin at all. I know my schematic is a bit confusing. key point is to transfer the energy (pressure & temp) from the bleed air to the ram air and blow out the bleed air instead of the ram air. everything else works just as a usual pack. this would prevent any leakage in the engine to enter the cabin

The DC8 used packs located in the nose to pressurize the cabin. Even though the air inlets were located in the chin it still needed turbocompressors to pressure the aircraft. You can't rely on ram air for pressurization because of the varying speed of the aircraft. Also if one of the two packs in the DC8 failed at FL 360 you could maintain normal pressurization on three turbocompressors. If you lost two turbocompressors you would have to descend a bit. If you had no turbo compressors and lost an entire pack you'de be landing. Take into account minimum enroute altitudes and you'll see the need for bleed air or turbocompressors.

Nah, you would never get the pressures or temperatures you need to run the ECS packs with ram air alone. I know above certain altitudes, you do get turbine by pass, which i think takes the ram air to augment the system. Even if you could get the pressures and temps to run ECS, your wing anti-ice system takes a HUGE bite out of the bleed air.

Quoting horstroad (Thread starter):it would just need a second air cycle machine to transfer the energy from the bleed air to the ram air

Exactly. Keeping the bleed air clean (totally possible, and normal, with good maintenance) means you can skip the weight of an entire extra air cycle machine and related ducting.

Quoting liedetectors (Reply 4):
Nah, you would never get the pressures or temperatures you need to run the ECS packs with ram air alone.

That's not what he's saying...the OP means use bleed air to power a turbocompressor to compress ambient ("ram") air into the cabin. That way you can't get any oil vapour from the engine into the cabin (a huge complaint for a few airlines, a total non-issue to most).

Quoting liedetectors (Reply 4):Nah, you would never get the pressures or temperatures you need to run the ECS packs with ram air alone. I know above certain altitudes, you do get turbine by pass, which i think takes the ram air to augment the system. Even if you could get the pressures and temps to run ECS, your wing anti-ice system takes a HUGE bite out of the bleed air.

Would you get to the pressure only? Heat could be supplied from waste heat in hydraulics and/or oil. I guess the weight of this probably exceeds the weight of the ACM or it would presumably be done.

Yes... bleed air is taken from the engines and run forward to the 4 turbo compressors and then to the packs. The 70 series planes did away wit the turbo compressors and just used bleed air to pressurize the cabin. On the 70 series you will see the 4 turbo compressor inlets blocked. The center inlet is for air used to cool the pack heat exchangers.

There was also a conversion for the 54/55/61 series as well in the 90's called the NASI system which also did away with the turbo compressors. But it never worked correctly. Crews got sick from oil contamination entering the cabin. Some crews refused to fly the planes with the NASI system.

The TC's required a lot of maintenance and failed fairly often. I scremed like a little girl when one exploded on us. It's located right below the pilots feet. It was loud. You could tel lwhen one was ready to fial on your pre-flight. The overboard TC turbine discharge along the fuselage would be covered in oil.